The molding of polymeric composites such as sheet molding compounds and bulk molding compounds is well known. Generally, such polymeric composites contain glass fiber reinforced polyester-based resins, filled or unfilled, which crosslink by a polymerization reaction which takes place most usually under heat and pressure. Such sheet molding compound material is commonly prepared and stored between flexible polymeric film layers in the form of plies on rolls or continuously folded into large storage containers for the duration of the chemical thickening or maturation period before being molded. In preparation for the molding process, the plies frequently are cut into the shapes and dimensions desired for proper placement into the mold. Although the plies may be molded individually, when objects of greater thickness than that produced by the thickness of an individual ply are to be formed, it is conventional to pile the plies in contiguous relationship and to mold the thicker object therefrom.
However, in such instances, and particularly when two or more plies are so used, and when all the plies are substantially identical, certain deficiencies in the molded part result. These involve the existence of curing seams between the various plies, the seams existing because no melding of the plies occurs at their contact points. This failure to meld occurs principally due to unlike curing initiation occurring among the individual plies inasmuch as the individual curing fronts proceed inwardly from each ply, or from the heated mold surface, with the result that when the individual curing fronts meet at or near the ply edges, a curing seam is created. The presence of such seam, or seams, weakens the molded part principally by providing stressed or slippage surfaces at such curing seams along which surfaces shear can occur; thus, the shear strength of the part is reduced.
Traditionally, compression molding of parts having cross sections greater than 0.25 inch was accomplished by varying the mold cycle time, molding temperature, preheating the SMC preform, or the cut and prepared charge, or by employing a post cycle cooling schedule. The decrease in mold cycle time, a prime factor in the part processing cost, is an attractive target. Unfortunately, in practice, a compromise is made between mold cycle time and the ultimate properties obtained in the thick section. In fact, effective translation of thin section properties, especially tensile and shear strengths, to the thick cross section is seldom accomplished.
There has now been discovered a solution to this problem. The solution involves employing as the plies a series of moldable polymeric composities formulated to "co-cure" by arranging the plies such that the material with the lowest polymerization initiating temperature is positioned centermost of the plurality of plies, with plies having progressively higher initiating temperatures being positioned outwardly therefrom in the order of increasing initiating temperature. Properly evaluated in terms of heat transfer and reaction initiation, when molded, all plies begin to cure at substantially the same time, and curing seams are substantially eliminated.